1. Field of the Invention
The present invention relates to a projection system for a charged particle multi-beamlet system, such as for a charged particle multi-beamlet lithography system or an inspection system, and an end module for such a projection system.
The present invention more specifically relates to a deflection device, in particular a beamlet blanker for use in a charged particle multi-beamlet lithography system, the blanker comprising a substrate provided with apertures and deflectors with a first electrode and a second electrode arranged around an aperture, the deflectors receiving electrical signals for deflecting beamlets of charged particles passing through the apertures.
The present invention further relates to a charged particle multi-beamlet lithography system, to a deflecting method and a method of transferring a pattern to a target surface.
2. Description of the Related Art
Currently, most commercial lithography systems use a mask as a means to store and reproduce the pattern data for exposing a target, such as a wafer with a coating of resist. In a maskless lithography system, beamlets of charged particles are used to write the pattern data onto the target. The beamlets are individually controlled, for example by individually switching them on and off, to generate the required pattern.
One type of design used for charged particle multi-beamlet systems is shown for example in U.S. Pat. No. 5,905,267, in which an electron beam is expanded, collimated and split by an aperture array into a plurality of beamlets. The obtained image is then reduced by a reduction electron optical system and projected onto a wafer. The reduction electron optical system focuses and demagnifies all the beamlets together, so that the entire set of beamlets is imaged and reduced in size. In this design, all the beamlets cross at a common cross-over, which introduces distortions and reduction of the resolution due to interactions between the charged particles in the beamlets.
Designs without such a common cross-over have also been proposed, in which the beamlets are focused and demagnified individually. However, when such a system is constructed having a large number of beamlets, providing multiple lenses for controlling each beamlet individually becomes impractical. The construction of a large number of individually controlled lenses adds complexity to the system, and the pitch between the lenses must be sufficient to permit room for the necessary components for each lens and to permit access for individual control signals to each lens. The greater height of the optical column of such a system results in several drawbacks, such as the increased volume of vacuum to be maintained and the long path for the beamlets which increases e.g. the effect of alignment errors caused by drift of the beamlets.
The controlled switching of beamlets may be achieved using a beamlet blanker. Controlled by control signals, a beamlet is deflected during a first period and continues without deflection during a second period of time. If deflected, the beamlet will terminate at a beam stop. If not deflected, the beamlet will pass the beam stop. In this manner, a “patterned” beamlet is generated, appearing after the beam stop. This patterned beamlet is subsequently projected onto a target surface by a projection lens system. For high resolution lithography systems designed to operate at a commercially acceptable throughput, the size, complexity, and cost of such systems becomes an obstacle.
One such beamlet blanker is known from Jpn. J. Appl. Phys. Vol. 32 (1993) Part 1, no. 12B, pp. 6012-6017. This beamlet blanker is embodied as an array for individual deflection of 1024 beamlets. The array comprises a silicon substrate with 1024 apertures, also referred to as through-holes, each in the form of a square of 25 μm×25 μm size. Wires connecting to blanking electrodes run between the holes. The highest wire density is seven wires between holes, each wire having a line width and spacing of 2 microns. Electrodes are provided by gold plating to a thickness of 40 microns. Manufacturing of such a beamlet blanker is based on a thermally oxidized silicon substrate. After wire patterning, the thermal oxide is etched. After the gold plating of the specified wires, the silicon substrate is etched from its bottom side. The first electrode is formed in a U-shaped design and operates as a ground electrode. The second electrode is block-shaped and acts as a switching electrode. The individual ground electrodes are mutually electrically connected. In operation, sixty-four bits of data for each blanking plate are read out in parallel from a DRAM each 160 ns and set in a shift register. These are then transferred over the individual wires to each of the switching electrodes in the array.
Existing charged particle beam technology is suitable for lithography systems for relatively course patterning of images, for example to achieve critical dimensions of 90 nm and higher. However, a growing need exists for improved performance. It is desired to achieve considerably smaller critical dimensions, for example 22 nm, while maintaining sufficient wafer throughput, e.g. between 10 and 60 wafers per hour.
The achievement of such smaller minimum feature size is directly related to a spot size of a single charged particle beamlet. For patterning a wafer with reduced spot size, more spots are needed to pattern a given surface area. For maintaining wafer throughput at reduced spot size, there is thus a desire of accelerating the patterning per spot.